Question: $ B = \left[\begin{array}{rrr}2 & 1 & 4 \\ 4 & 3 & 5\end{array}\right]$ $ E = \left[\begin{array}{rr}3 & -1 \\ -2 & 4 \\ -1 & 4\end{array}\right]$ What is $ B E$ ?
Explanation: Because $ B$ has dimensions $(2\times3)$ and $ E$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ B E = \left[\begin{array}{rrr}{2} & {1} & {4} \\ {4} & {3} & {5}\end{array}\right] \left[\begin{array}{rr}{3} & \color{#DF0030}{-1} \\ {-2} & \color{#DF0030}{4} \\ {-1} & \color{#DF0030}{4}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ E$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ E$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ E$ , and so on. Add the products together. $ \left[\begin{array}{rr}{2}\cdot{3}+{1}\cdot{-2}+{4}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ E$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{3}+{1}\cdot{-2}+{4}\cdot{-1} & ? \\ {4}\cdot{3}+{3}\cdot{-2}+{5}\cdot{-1} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ E$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{3}+{1}\cdot{-2}+{4}\cdot{-1} & {2}\cdot\color{#DF0030}{-1}+{1}\cdot\color{#DF0030}{4}+{4}\cdot\color{#DF0030}{4} \\ {4}\cdot{3}+{3}\cdot{-2}+{5}\cdot{-1} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{2}\cdot{3}+{1}\cdot{-2}+{4}\cdot{-1} & {2}\cdot\color{#DF0030}{-1}+{1}\cdot\color{#DF0030}{4}+{4}\cdot\color{#DF0030}{4} \\ {4}\cdot{3}+{3}\cdot{-2}+{5}\cdot{-1} & {4}\cdot\color{#DF0030}{-1}+{3}\cdot\color{#DF0030}{4}+{5}\cdot\color{#DF0030}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}0 & 18 \\ 1 & 28\end{array}\right] $